The two stacked Figure 1 panels (top-corn; bottom-soybean) show NASS weekly planting progress estimates (solid symbols) for each of four notable past years (1982 - red, 2012 - black, 2019 - green, and 2020 - blue). Planting progress typically follows a Sigmoid (S-shaped) curve, from inception (0%) to completion (100%), and thus can be modeled using a Logistic Equation (described in the graph text boxes). Progress is initially slow, but gradually accelerates until reaching a 50% progress inflection mid-point (depicted by thick central horizontal line in each graph), after which it gradually decelerates upon its approach to final completion.

The equation was used to compute a Logistic Xm value — demarking the spring date when each colored curve reached its 50% progress point. The colored vertical lines point to the Day of Year (graph bottom axis) or Month-Day (top axis) when that crossover occurred. A Logistic k rate value was also estimated for each curve. It is a numerical indication of planting progress speed. A steeper curve is generated with a faster k rate (e.g., 1982 soybean curve), whereas a flatter curve results from a slower k rate (e.g., 2019 corn and soybean curves). The grey band bracketing each curve is a statistical 95% confidence interval (CI) that reflects the intrinsic degree of random error associated with the estimation of planting progress. Error-free trend estimation is never possible, so successive data points would rarely, if ever, perfectly track a modeled curve trend line.

The four curves highlighted in Figure 1 were chosen to be representative (to date) of the earliest, latest, fastest and slowest of each crop’s 43-year set of curves. For example, the latest 50% date of corn progress was attained in a slow 1982 (May 24) curve, whereas the latest soybean 50% date was attained 19 days later in the fastest ever 1982 (June 12) curve. Alternatively, the earliest 50% date of corn progress was attained in the 2012 (May 2) and 2020 (May 3) curves, whereas the earliest 50% soybean date was attained a respective eight to nine days later in the 2020 (May 10) and 2012 (May 12) curves.

The Figure 2 stacked panels display all 43 years of the Logistic-modeled curves of NASS weekly planting progress estimates. Differing curve coloration of the first six years (1980-1985) versus the last six years (2017-2022) helps to reveal that the month-day of 50% planting progress was, on average, much later in the “bygone days” versus “nowadays”.

The two stacked graphs also provide a comparison of the left-to-right “dispersive spread” of each crop’s set of 43 curves. All but five of the corn progress curves had 50% dates concentrated within a May 2-12 (10-day) time span, whereas all but eight of the soybean progress curves had 50% dates occupying a later May 12-25 (13-day) time span. The staggered difference between these two respective “planting windows” is a consequence of most Nebraska producers being motivated to first plant all their corn fields before planting any soybean fields.

However, that “corn-first” sentiment has moderated in recent years. Indeed, a significant fraction of corn-soybean producers now plant soybean either before or concurrently with corn planting. This is inferably evident when one views the leftward shifted positions of the starting and accelerating phases of the 2020, 2021 and 2022 soybean plant curves (see the bottom left quadrant of the soybean graph).

Notably, the most recent soybean progress curves have a less steep shape (i.e., slower k rate). A slower recent progress rate would seem to be counter-intuitive, but it results from an earlier start of soybean planting by those Nebraska producers motivated to plant soybeans before (or with) corn, followed by a later start of soybean planting (and completion) by the many Nebraska producers who have not yet committed to implementing that change. As a result, the front half of the curve is advanced forward by the foregoing producers, whereas the back half of the curve is not. This mathematically leads to a less steep curve (i.e., slower curve k value).

The Figure 3 stacked panels depict graphs of the spring date of 50% planting progress (circle symbols) associated with each year’s Logistic-modeled curve from 1980-2022. For reference, the maximum, minimum, and mean spring date values are shown as dashed horizontal lines in each crop’s graph.

In the corn graph, it is evident that no advancement of Nebraska corn 50% progress has occurred in the past three decades. Simple linear regression did detect a statistically significant, but very minimal (-0.17 day per year; dashed green line) trend towards earlier corn planting. However, a linear-plateau regression model was statistically more probable (based on the Akaike Information Criterion — AIC test). Thus, May 7, 1989 could serve as a useful baseline prediction for future spring dates of 50% progress in Nebraska planted corn.

In that regard, Nebraska producers could use May 7 as a benchmark spring date guideline relative to aiming for a 50% completion of corn field planting on their farms. Note, however, that since 2000, spring dates for 50% corn planting progress have varied from as early as May 2 (2012) to as late as May 13 (2019). For the present year, we note that a NASS May 7 estimate of 2023 Nebraska corn planting progress will be released this coming Monday.

In contrast, the soybean graph shows that Nebraska producers have markedly advanced the 50% progress date over the past four decades. Simple linear regression indicated an annual hastening of the spring date by 0.38 days per year, leading to a projection of May 14 for the spring date in the year 2023. The less probable (as per the AIC test) linear-plateau model suggested a faster spring date advance (0.46 days per year), but only up to 2011, thereafter leading to a flat projection of May 17 for future soybean spring dates. Notably, if the late 2019 spring date is treated as an extreme outlier data point, and if omitted from regression analyses, the linear-plateau model fit becomes even less probable.

In any event, Nebraska producers could choose to use either May 14 or May 17 as a future benchmark spring date goal for attaining 50% completion of soybean field planting on their farms, though again noting the variance from as early as May 10 (2020) to as late as May 25 (2019). Again, see next Monday’s release of the NASS May 7 estimate of 2023 Nebraska soybean planting progress.

The Figure 4 panels display graphs with a vertical yield axis and a horizontal 50% spring date axis (bottom Day of Year, or top Month-Day). Within each graph are shown the data points that pair each year’s NASS-estimated crop yield for Nebraska Irrigated-○ (blue), Rainfed-○ (brown), & Both-□ (i.e., combined IR&RF, represented in green) production systems with the same year’s NASS-estimated 50% planting progress date. Note that the two graphs were constructed to have matching bottom DOY and top Month-Day axis coincidence to more clearly highlight the difference in the spring planting windows that producers have typically allocated to corn (prior to May 15) and to soybean (generally thereafter) over this 43-year historical record.

In both graphs, only the recent year 2011 to 2018 IR data points are year-labeled — readers can easily gaze straight down to find the corresponding same-year (non-labeled) RF and Both data points. Note: NASS ceased providing IR and RF data after 2018, so year labels for 2019 to 2022 are depictable only for the Both data points. The downward sloping IR, RF, Both solid trend lines were derived from linear regression analysis of the respective same colored data points (see the upper right text boxes).

In the corn graph, the IR trend line indicates that just a one-day delay in the 50% progress date could be expected to lessen the resultant IR corn yield by a statistically significant 3 bu/ac, whereas the RF trend line displayed a (barely) significant 1.8 bu/ac per day of delay in the 50% date. However, when the five extremely late 50% corn plant date years (i.e., 1982-1984 and 1994-1995) enclosed in the dotted box in lower right corn graph were omitted from the regression analyses (as conjectured outliers), all three re-computed (dashed) trend lines (IR, RF, Both) were judged to be NOT statistically significant (P>0.05).